1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) transceiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscribers' subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that maybe caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF transceivers. Radio frequency transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts it to IF and then to baseband. In other systems, the received RF is converted directly to baseband.
In the initial signal processing stages of an RF receiver, the received RF signal is converted to baseband through one or more steps. Initially, the received RF is mixed with a local oscillator (LO) to down convert the carrier frequency to baseband. It is common to utilize a low pass filter coupled to the output of the mixer to remove introduced interference. One problem with using a mixer and low pass filter, however, is that both add a fixed amount of gain to the received signal, irrespective of the received signal strength. This amplified signal is passed to the baseband processor, which is sensitive to excessive signal strength that can cause quantification errors in downstream analog-to-digital converters (ADCs) and saturation in the output devices. One approach that may be used is to limit the amount of power received by the mixer and low pass filter by adjusting the gain of the receiver's low noise amplifier (LNA) according to a received signal strength. This approach, however, requires a method to detect the peak amplitude of the received signal then provide a gain control signal to the receiver's LNA that is proportional to the received signal amplitude.
Since a received signal was transmitted using common modulation techniques that include I and Q signal components, one method of detecting the peak amplitude is to take the square root of the sum of I squared and Q squared [(I2+Q2)1/2]. The square root function is traditionally performed using a digital signal processor (DSP) in the digital domain on baseband signals. Unfortunately, the DSP function requires processing time and, in some cases, may not respond fast enough to avoid saturation or, alternatively, under-amplification of the output stages.
There is a need in the art, therefore, for a circuit and a method to detect a peak amplitude of a modulated multi-channel signal. Some designs have attempted to satisfy this need by developing an analog system that produces an output that is proportional to the sum of the logarithms of the I and Q modulated channels' amplitude components. In a theoretical world, such an approach is satisfactory. In a real world, however, because oscillators that are used to down convert received RF include crystals that are not perfect and tend to vary from a specified frequency, they introduce a frequency error that effectively adds a modulation component to the logarithms of the I and Q modulated channels' amplitude components.
This modulation component tends to cause the peak amplitude determination to fluctuate thereby causing amplifiers whose gain is adjusted in response to detected peak values of a signal to fluctuate in a corresponding manner. Accordingly, there is a need for an analog peak amplitude detector that eliminates or minimizes the effects of the modulated components that are added due to frequency errors introduced by upstream devices.